The present invention relates to the control of the production of sulfites, of hydrogen sulfide and of acetaldehyde during alcoholic fermentation by yeasts.
Sulfur dioxide (SO2) and its various forms in equilibrium in solution (HSO3−, SO3−), collectively denoted sulfites, are used as additives in enology, principally to improve the conservation of wines, owing to its antioxidant and antibacterial properties. However, an excessive amount of sulfites in wine can lead to intolerances and allergies in certain consumers; they may also be prejudicial to its organoleptic qualities, given that they give, if there in excess, drying sensations. Excessive amounts of sulfites at the end of alcoholic fermentation can thus be disadvantageous when the wine producer wants to carry out malolactic fermentation. Lactic acid bacteria, responsible for this fermentation, are inhibited by low sulfite contents, and an excess delays the initiation of said fermentation. Hydrogen sulfide is also a metabolite formed by yeasts in fermentation which is prejudicial to the quality of wines when it is present in excess owing to the “rotten egg” or “reduced” tastes that it imparts.
It is therefore important to be able to optimize the amount of sulfites and of hydrogen sulfide in wines and during winemaking. A major difficulty in this context comes from the fact that part of the sulfites and of the hydrogen sulfide present in the wine comes from the fermentative metabolism of yeasts, where they constitute intermediates in the synthesis of sulfur-containing amino acids. Inorganic sulfate enters the cell by means of a sulfate permease. It is activated to give adenosylphosphosulfate (APS) by ATP-sulfurylase, then the APS is phosphorylated by adenosylphosphosulfate kinase to produce phosphoadenosylphosphosulfate (PAPS). The PAPS is then reduced to SO2 by PAPS reductase. The SO2 is reduced to H2S by sulfite reductase. Homocysteine, which is the precursor of sulfur-containing amino acids, is synthesized by reaction of H2S with O-acetylhomoserine, catalyzed by O-acetylhomoserine sulfhydrylase.
Since the amount of sulfites produced by yeasts during fermentation varies from one yeast strain to another, this complicates the control of the overall sulfite content. The same is true for hydrogen sulfide, the amount of which formed depends greatly on the yeast strain.
Another compound, the presence of which in wine above certain amounts is considered to be undesirable, is acetaldehyde. Acetaldehyde at too high a concentration gives wines “musty” notes which are considered to be negative. It is produced by yeasts during fermentation, and its production appears to correlate with the SO2 content, and like that of the SO2, varies from one yeast strain to another.
Various approaches have been proposed for obtaining yeast strains producing reduced amounts of sulfites and/or of hydrogen sulfide.
PCT application WO 2008/115759 and PCT application WO 2009/046485, and also the publications by Cordente et al. (FEMS Yeast Res, 9, 446-59, 2009) and Linderholm et al. (Appl Environ Microbiol, 76, 7699-707, 2010), describe various mutations in the METS or MET10 genes (encoding the 2 catalytic subunits of sulfite reductase) which have the effect of reducing hydrogen sulfide production. Application WO 2009/030863 and the publication by Marullo et al. (FEMS Yeast Res, 7, 1295-306, 2007) describe various markers associated with characteristics of interest in enological yeasts. One of these markers (YOL083w) located on chromosome XV is associated with a reduced H2S production.